Fluorescent lamps are becoming increasingly popular in both commercial and residential applications. Fluorescent lamps are more energy efficient and last longer than traditional incandescent lights.
The visible light from a fluorescent lamp is produced by a mixture of phosphors inside the lamp. They give off light when exposed to ultraviolet radiation released by mercury atoms as they are bombarded by electrons. The flow of electrons is produced by an arc between two electrodes at the ends of the lamp.
The ambient temperature around a fluorescent lamp can have a significant effect on light output and lamp efficiency. The temperature of the coldest spot on the surface of the lamp is where mercury vapor will condense to liquid form, and this temperature (the “minimum lamp wall temperature”) controls the vapor pressure inside the lamp. The optimum lamp wall temperature for fluorescent lamps ranges from about 10° C. to about 75° C., depending on the ambient temperature. At temperatures below the optimum, mercury vapor will condense at the cold spot, reducing the number of mercury atoms available to emit UV radiation. At temperatures above the optimum, an excess of mercury vapor is present, absorbing the UV radiation before it can reach the phosphors. In both cases, light output drops.
Not all fluorescent systems are equally susceptible to low-temperature problems, but in general, as temperature drops, so do light output and efficiency. At very low temperatures (below 32° F. or 0° C.), lamp output can decline to one-third the rated value or less. It is important to note that some fluorescent lamps will have to warm up a while before producing sufficient light under cold conditions, some may take several minutes to ignite, and some won't start at all.
For cold applications (either indoors or out), fluorescent lamps and ballasts can be designed specifically for low-temperature operation. These lamps are usually equipped with electronic ballasts and can be enclosed in globes or recesses to prevent wind chill of the lamp. Even with these precautions, the lamp will most likely not operate at the same efficiency and produce the same amount of light as it would under more hospitable ambient conditions. However, the heat produced by the lamp during use will generally be enough to increase the temperature of the lamp enough to make the lumen output sufficient.
High ambient temperatures may be produced around enclosed fluorescent lamps in interior lighting applications. In addition, less-efficient ballasts will introduce more heat into fixture enclosures. The IES Lighting Handbook points out that a 1% loss in light output (for fluorescent lamps in general) can be expected for every 2° F. (1.1° C.) above the optimum ambient temperature. Efficiency can also drop, to some degree, at these higher temperatures.
Some current state of the art lighting assemblies are equipped with heat dissipating devices that are designed to cool the fluorescent lamp housed within the lighting assembly to an optimum temperature. A problem with at least some of these devices is that the heat dissipating devices, when available, are always in cooling mode. Thus, for all ambient temperatures, the heat dissipating devices remove heat. Often, the cold spot of the fluorescent lamp is cooled below the optimum temperature. Therefore, what is needed is a fluorescent lighting assembly equipped with a heat dissipating device that keeps the fluorescent lamp within the desired temperature range across a larger range of ambient temperatures.